A manufacturable, high resolution photoresist process is essential for the development of VLSI circuits. Single layer photoresist processes known in the art are capable of fine resolution when used on smooth, low reflectivity substrates. Unfortunately, the process of manufacturing integrated circuits creates a complex surface of widely varying reflectivities. This has mandated the development of alternative photoresist processes.
One such process is the portable conformal mask process (PCM). This process is a two layer resist process. The first resist layer is applied in sufficient thickness to planarize the substrate. The second resist layer is a standard resist layer, which is applied in uniform thickness. Then, the second resist layer is exposed and developed. Finally, the first resist layer is exposed and developed. With the mask in place, the chemical processing steps begin. One common process involves etching silicon dioxide which is used as an insulator in the construction of integrated circuits.
Silicon dioxide, or glass, is very difficult to etch. Ammonium fluoride has been the primary ingredient in all silicon dioxide etchants. In order to increase the pH stability of such etchants, the prior art has taught the addition of two ingredients, either hydrogen flouride or acetic acid.
However, severe line width problems are encountered when films of phosphorous doped or undoped silicon dioxide masked with the PCM technique are etched with these standard buffered etch solutions. Use of the standard etch caused the first resist layer to lift, thereby permitting the etchant to etch portions of the silicon dioxide film which were not intended to be etched. Furthermore, the standard etchant also caused severe undercutting which prevents the accurate control of line widths required for VLSI circuits.
Prior art solutions to such problems have included: (1) attempting to improve resist adhesion to the silicon dioxide; (2) formulating the best developer conditions; (3) determining the optimum hard bake temperature; and (4) varying the concentration of ammonium fluoride and acetic acid or hydrogen fluoride in the etchant solution. Each of these attempted solutions has failed to resolve the problem of maintaining very narrow line widths to the high degree of uniformity and accuracy demanded by the VLSI process.